A number of different kinds of borehole seismic sources have been utilized for generating a seismic signal in a borehole, including impulsive sources, such as primacord and air guns. Swept frequency signal generators have also been employed including the source disclosed in U.S. Pat. No. 4,671,379, and sources using piezoceramic actuators, such as disclosed in U.S. patent application Ser. No. 09/002,474, having a filing date of Jan. 2, 1998.
Piezoelectric actuators typically comprise a plurality of cylindrical elements of a piezoceramic material which are aligned so that the elements operate in unison. These aligned piezoceramic elements are then enclosed in an elongated bladder or sleeve which is substantially filled with a fluid. Such an elongated sleeve or bladder isolates the piezoceramic elements, and the accompanying electrical conductors, from the borehole fluid. Enclosing the aligned piezoceramic elements in an elongated expandable bladder also assists the source in achieving a resonance mode, which enhances the output power generated by the source. Maximum power output is achieved at a resonance frequency, such as the half wave resonance frequency. The half wave resonance frequency, f, is defined by the equation: EQU f=c/2L
in which c is the acoustic velocity of the fluid which fills the bladder or sleeve, and L is the internal length of the bladder or sleeve.
In the prior art, the bladder has typically been formed from an elastomeric material, because elastomeric materials expand and contract readily. An expandable bladder or sleeve increases the magnitude of the generated signal at resonance. A severe problem has been encountered with the use of elastomeric bladders in downhole operations, however. Gases that are present in the borehole fluid will penetrate the bladder and collect in the interior of the bladder. When the piezoceramic source is brought back to the earth's surface, the gases inside the bladder expand and the elastomeric bladder can rupture or explode. The elastomeric material itself can also be damaged because it can absorb borehole gases which will cause the material itself to rupture when it is brought back to the surface.
To alleviate the problem discussed above with respect to elastomeric material, it has been proposed in U.S. patent application Ser. No. 09/002,474, having a filing date of Jan. 2, 1998, to use a cylindrical metallic sleeve, rather than an elastomeric bladder. However, cylindrical sleeves have limited ability to expand and contract and the power that can be generated at a resonance frequency is limited accordingly.
It is an object of the invention to achieve a borehole source that does not employ an elastomeric bladder and that achieves a higher output power than a cylindrical metallic sleeve.
U.S. Pat. No. 4,671,379, which issued to Kennedy et al. on Jun. 9, 1987 and U.S. Pat. No. 4,834,210, which issued to Kennedy on May 30, 1989, disclose a system in which a pulsed energy source is deployed between two end members. Wellbore fluid occupies the space between the two end members, and the pulsed energy source excites the fluid into oscillation within the borehole space between the two end members. The distance between the two end members is varied as the frequency of the pulsed energy source is varied in order to maintain the system at resonance.
U.S. Pat. No. 5,171,943, which issued to Balogh et al. on Dec. 15, 1992, discloses a tube wave damper probe for the suppression of borehole tube waves in seismic applications. The damper comprises a gas-filled bladder within a housing. The bladder is filled with gas before the bladder is inserted into the borehole.
S. T. Chen, "A Single-Well Profiling Tool and Tube Wave Suppression" Expanded Abstract, SEG, 13-16, 1993, discusses an acoustic source comprising a stack of hollow PZT cylinders driving two end hemispherical masses.
W. T. Balogh, "The Borehole Tubewave Damper Probe" Expanded Abstracts, SEG, 159-162, 1992, describes a tube wave damper probe which utilizes a gas-filled bladder to attenuate borehole tube waves.
U.S. Pat. No. 5,137,109, which issued to Dorel on Aug. 11, 1992, discloses a downhole seismic source in which a body containing a resonant system is clamped to the borehole wall. The body contains a resonant system and excitation means for exciting the resonant system at an excitation frequency which is tuned to the resonance frequency. The source includes means for varying the resonance frequency of the resonant system in a controlled manner while actuating the excitation means. In this system the seismic signal is applied to the borehole by clamping the source to the borehole wall.
U.S. patent application Ser. No. 09/002,474, filed on January 1998 discloses a dual mode multiple-element resonant cavity piezoceramic borehole energy source utilizing the resonance frequency of the piezoceramic cavity for higher output at lower frequencies and a nonresonant mode extending across a broader frequency range at higher frequencies. The piezoceramic crystals are housed within a metallic bladder. The metallic bladder has the advantage that it is able to withstand the corrosive environment of the borehole. However, there is a need for a system that has a greater ability to transmit energy from the piezoceramic source into the borehole fluid than the metallic bladder proposed in this patent application.